1. Field of the Invention
This invention relates to means for preventing collisions between automatically controlled vehicles (or trains) operating on a guideway (or track).
2. Description of the Prior Art
The traditional and universally used means for preventing collisions between vehicles operating on a guideway is the block control system. Such a system acts to command an appropriate speed reduction in a vehicle which approaches too closely to another vehicle ahead of it on the guideway.
A block control system divides the guideway into contiguous zones called blocks. When a vehicle enters a block, the block is said to be occupied and the block is instrumented to register such occupation. Instrumentation causes a vehicle entering specified blocks behind an occupied block to receive "slow-down" or "stop" commands to insure that a collision between leading and trailing vehicles does not occur.
The advent of "people mover" or "personal rapid transit" systems, in which automatically controlled vehicles operate with nominal time separations (headways) much shorter than those of traditional rapid rail transit trains, has resulted in departures from traditional block control means for preventing vehicle collisions. For very short headways, several seconds or less in length (as contrasted with rapid rail headways which generally exceed one minute), block control systems require very large numbers of blocks per unit length of guideway. Since each block has an irreducible complement of hardware components, a control system for a very-short headway transit system would be inordinately complex and expensive.
To avoid the difficulties inherent in application of block control to vehicles moving at very-short headways, development activity for collision avoidance systems applicable to such vehicles has been concentrated on accurate measurement of intervehicle distance. Safe vehicle separation is assured by initiating emergency braking when measured intervehicle distance is less than some predetermined threshold. One such means of intervehicle distance measurement involves the use of radar techniques wherein a high-frequency electromagnetic signal transmitted from a trailing vehicle is reflected from a leading vehicle, and the intervehicle distance is inferred from the measured delay between the transmitted signal and the reflected signal received in the trailing vehicle. However such radar means for measuring distance have not been successful for reasons as detailed hereinafter.